(i) Field of the Invention
The present invention relates to a wafer cleaning apparatus and a structure for holding and transferring a wafer used in the wafer cleaning apparatus. In particular, it relates to a wafer cleaning apparatus in which the surface of a transferred wafer is cleaned with particles of ice obtained by jetting a cleaning liquid through a nozzle at a supersonic speed, and a structure for holding and transferring a wafer used in the wafer cleaning apparatus.
(ii) Description of the Related Art
In manufacturing ULSIs, the yield and the quality of the products are remarkably lower if contaminants such as particles adhere to the surface of a wafer during the manufacturing step. For this reason, it is necessary to make the efficiency of the technique for cleaning the wafer surface higher. Recently, in the semiconductor industry, the influence of contaminants, such as particles, on the yield of the products has become considerable because of the demand for larger-scale integration on the silicon wafer.
The present applicant has already filed Japanese Patent Application No. Hei9-19375 as a highly efficient cleaning technique for cleaning the surface of a wafer. A wafer cleaning apparatus disclosed therein is provided with a jet orifice such as a Laval nozzle, through which minute particles of ice are jetted to the wafer surface to remove adhering contaminants.
FIG. 7 is a perspective view of such a semiconductor wafer cleaning apparatus for cleaning the surface of a wafer with ice particles jetted. As shown in FIG. 7, this semiconductor wafer cleaning apparatus comprises a transfer robot 32 disposed near the center of the apparatus, for transferring a wafer 31. The wafer 31 is loaded on the transfer robot 32 and unloaded from it. The apparatus also includes a loader 33 for receiving the wafer 31 before cleaning, an unloader 34 for receiving the wafer 31 after cleaning, an alignment 37 disposed between the loader 33 and the unloader 34 for adjusting the position of the wafer 31, and a cleaning section 35 for cleaning the wafer 31. The cleaning section 35 includes a spinner 42 for holding the wafer 31 by vacuum chuck and rotating it, a nozzle 40 for jetting a cleaning liquid to the surface of the rotating wafer 31 to clean it, and a discharge cup 44 surrounding the periphery of the spinner 42 so as to prevent the cleaning liquid jetting through the nozzle 40 from flying in to the surroundings. Operation buttons 38 are provided near the loader 33 for operating the apparatus in a predetermined cleaning condition. A personal computer 39 is provided near the operation buttons 38 for analyzing the cleaning condition and displaying the result.
The transfer robot 32 is disposed near the center of the apparatus and provided with a chuck portion for chucking the wafer 31 by vacuum. The chuck portion of this transfer robot 32 is movable between the front and the rear. The transfer robot 32 is rotatable around a predetermined axis in 360.degree.. The transfer robot 32 is thus designed so that it can hold the wafer 31 by vacuum chuck and transfer the wafer 31 to any of the loader 33, the unloader 34, the cleaning section 35 and the alignment 37, which are provided in the periphery, for performing a cleaning operation.
To the loader 33 provided in the periphery of the transfer robot 32, a wafer cassette (not shown), for receiving a plurality of wafers 31 before cleaning, is transferred from a predetermined section. Another wafer cassette (not shown), for receiving the wafers 31 after cleaning, is put in the unloader 34. The alignment 37 is provided between the loader 33 and the unloader 34. The alignment 37 is for detecting the center of the wafer 31 loaded by the transfer robot 32 and performing the centering adjustment of the wafer 31. The cleaning section 35 is provided near the loader 33 for cleaning the wafer 31.
The cleaning section 35 is provided with the spinner 42 for chucking the wafer 31 transferred from the loader 33 by being chucked by the transfer robot 32. The spinner 42 for chucking the wafer 31 is supported so as to be rotatable and engaged with a drive source, such as a motor. In this manner, the center of the wafer 31 is detected by the alignment 37, and the wafer 31 is centered. The wafer 31 is then fixed to the spinner 42 in the state that the center of the wafer 31 coincides with the center of the spinner 42 which is supported so as to be rotatable. The cleaning section 35 includes the nozzle 40 for cleaning the wafer 31 by jetting the cleaning liquid to the wafer 31 which is chucked by the spinner 42 and rotating with the spinner 42.
A through hole (not shown) is formed in the nozzle 40 for supplying compressed air from an air supply apparatus (not shown) provided in the exterior. The through hole is tapered toward the tip end near the bottom to form a Laval nozzle. The through hole of the nozzle 40 is designed so that pure water can be supplied in the middle of the through hole. The pure water is supplied to the through hole with a predetermined hydraulic pressure. In this nozzle 40, pure water and compressed air are supplied to the through hole, and the pure water is jetted through the tip of the nozzle 40 having the shape of Laval nozzle with a predetermined hydraulic pressure. As a result, at the tip of the nozzle 40, while the flow speed of the pure water increases, the pressure drops rapidly. The pure water is thus adiabatically expanded, and the temperature drops. The pure water supplied to the tip of the nozzle 40 thus transforms into ice particles because of the temperature drop. The ice particles are then jetted out of the tip of the nozzle 40. Under the nozzle 40 through which the ice particles are jetted, the wafer 31 chucked by the spinner 42 and rotating with the spinner 42 is located. The cleaning is carried out by making the ice particles collide to the surface of the wafer 31.
In the cleaning section 35, the discharge cup 44 surrounding and covering the periphery of the spinner 42 is provided so as to prevent the cleaning liquid jetted through the nozzle 40 from flying in to the surrounds. The discharge cup 44 has an opening in the upper surface so that the transfer robot 32 transferring the wafer 31 to the spinner 42 can easily perform the loading or unloading operation.
The operation buttons 38 for operating the apparatus are provided near the loader 33. The apparatus can be controlled by selecting the cleaning condition for the wafer 31 through the operation buttons 38. The wafer 31 is cleaned in the apparatus in accordance with the cleaning condition selected through the operation buttons 38. The personal computer 39 is provided near the operation buttons 38. The personal computer 39 is for displaying information obtained by simulating and analyzing the cleaning operation. The apparatus can therefore be operated in a predetermined condition, in light of the condition that is input through the operation buttons 38, on the basis of information obtained through the personal computer 39.
Next, the operation of cleaning the wafer in the wafer cleaning apparatus illustrated above will now be described. First, a wafer cassette, which received a plurality of wafers 31 before cleaning, is transferred from a predetermined section and put in the upper portion of the loader 33. Another wafer cassette, which received no wafers is put in the upper portion of the unloader 34. After placing the wafer cassettes, a predetermined condition is input through the operation buttons 38 on the basis of information obtained through the personal computer 39, and then the apparatus is started. After starting the apparatus, the transfer robot 32 moves toward the loader 33 to take out a wafer 31 from the loader 33 by vacuum chuck and transfer the wafer 31 to the alignment 37.
The wafer 31 transferred to the alignment 37 is centered by detecting the center of the wafer 31 and then transferred to the upper portion of the spinner 42 by the transfer robot 32 to be chucked so that the center of the wafer 31 coincides with the center of the spinner 42. After chucking the wafer 31, the spinner 42 is rotated, and pure water and compressed air are supplied to the nozzle 40. The pure water supplied to the nozzle 40 is thereby pressurized by the compressed air and accelerated at the tip of the nozzle 40 and then jetted out of the tip. Near the nozzle 40, the pure water is adiabatically expanded to drop the temperature, and as a result, ice particles are formed.
Under the nozzle 40 through which the ice particles are jetted, the wafer 31 rotating with the spinner 42 is located. The cleaning is carried out by making the ice particles collide to the surface of the wafer 31. The nozzle 40 is moved from the center to the circumference of the wafer 31 as the ice particles are jetted from above the rotating wafer 31. The wafer 31 is thus cleaned by blowing away contaminants such as particles.
After completing the cleaning of the wafer 31, the rotation of the spinner 42 and the jet from the nozzle 40 are stopped. At the same time, the transfer robot 32 chucks the wafer 31 on the spinner 42 and then transfers it to the wafer cassette in the unloader 34. The wafer cassette receives the wafer 31. By repeating such a cleaning operation as described above, all of the wafers 31 received in the wafer cassette in the loader 33 are cleaned.
In this manner, the wafer cleaning apparatus shown in FIG. 7 effectively cleans a semiconductor wafer by removing contaminants from the surface of the wafer with ice particles jetted through the nozzle.
In this wafer cleaning apparatus, however, a high rate of cleaning can not be obtained for removing stable contaminants, such as alien substances and metallic contaminants adhering to the wafer surface, because the cleaning liquid to be jetted consists of only pure water.
Further, because the cleaning is performed in the state of chucking the back surface of the wafer by the spinner 42, it is hard to remove all the contaminants using the jet through the nozzle, if the back surface of the wafer chucked is contaminated. It is also hard to clean the wafer after it has turned over.
Additionally, the transfer robot 32 accumulates the contaminants by chucking the wafer. Hence, there is a possibility that a newly chucked wafer is contaminated.
Furthermore, even in case of providing the discharge cup, it is difficult to fully discharge minute liquid drops which collided against the wafer surface and scattered. As a result, there is a possibility that the contaminated liquid again adheres to the wafer.